Bone screws are used for a multitude of purposes, including attaching various medical devices to bone. One such application involves the use of bone screws to attach a plate to adjacent bone structures or fragments to promote fusion of the structures or fragments into a unitary structure.
As an example, bone screws can be used to attach an anterior cervical plate to the relevant portion of a patient's anterior cervical spine to achieve fusion. Anterior cervical spinal fusion is a common approach for the surgical management of cervical disk disease, in which two or more vertebrae are brought together under conditions whereby the vertebrae fuse together to form a unitary member of the spinal column. Frequently, if there is significant spinal cord compression or if there is more than one disk level involved, a small plate is affixed on the anterior surface of the cervical vertebrae to provide greater permanent stability. In carrying out the procedure, the members must be brought together under conditions that are critically controlled to prevent infection, maintain alignment of opposing members, and allow for the stress in the bone that is generated as the healing process matures. According to the present art, the plates commonly used for cervical spinal fusion are fabricated from titanium with holes through which screws are inserted to secure the plate to neighboring vertebrae.
An additional application of bone screws in the spinal field is in the stabilization of the lumber spine. In one typical procedure, a bendable rod is attached by bone screws to various vertebrae along a portion of the patient's spine. Such a rod may have apparatus, including holes, configured to receive bone screws to secure the rod to the patient's vertebrae.
As another example, bone screws can be used to attach sternal closure plates to a patient's sternum following a median sternotomy. Many surgical procedures require a median sternotomy, a procedure in which an incision is made along the sternum such that it can be separated laterally into two sides, to allow access to the tissues and organs located in a patient's thoracic cavity. Following the surgical procedure, the two sides of the sternum must be secured together, which may be accomplished by attaching a plating system to them. The plating systems used for sternal closure may have holes through which bone screws are inserted to secure the plating system to the two sides of the sternum.
It has been recognized that osseous trans-differentiation during the course of bone healing may have a significant role in improving the structural integrity of healed bone following surgery or trauma. This process of bone remodeling may, however, result in partial extrusion of the bone screws placed during surgery to fix the position of bones or fragments if the fusion does not take place. Post-operative screw extrusion can be dangerous and may require further surgery. Locking the screws into place to disallow any degree of post-operative movement has been found to be undesirable because it precludes trans-differentiation, but it can be important that the movement of the screws be somewhat restricted to prevent more than slight screw extrusion.
The design of the bone screw is important, as it affects the speed and ease of implantation during surgery, the stability of fastening following surgery, and the potential for screw extrusion, among other characteristics. For example, some prior art screw designs require a drilling operation and/or a tapping operation to prepare the bone for the screw prior to its insertion. Such operations have numerous disadvantages, including the danger associated with introducing an additional instrument into the spinal canal during surgery, the potential for fracture formation, and the removal of bone material. Further, shearing of the thread crests during insertion has been experienced with some prior art screw designs, leading to the release of tiny metal shards in the patient. Further, some prior art screw designs have caused problems with loosening, breaking, and extruding from the bone. Again, this can create a dangerous condition that requires further surgery. Therefore, it is desirable to have a new and improved bone screw.
In addition to the design of the bone screw alone, the design of the bone screw in relation to a corresponding medical device, such as an anterior cervical plate or sternal closure plate, can affect the potential for screw extrusion. It is desirable to have a corresponding bone screw and plate system that allows only a beneficial degree of post-operative screw extrusion.